1. Field of the Invention
The present invention relates generally to cellular wireless communications and more particularly to the control and operation of common reverse channels in a code division multiple access (CDMA) cellular wireless communication system.
2. Description of the Related Art
Cellular wireless communication systems are generally known to include a plurality of base stations dispersed across a geographic service area. Each of the base stations includes at least one antenna and a base station transceiver system (BTS) and provides wireless service within a respective cell. The BTSs couple to base station controllers (BSCs) with each BSC serving a plurality of BTSs. Typically, the BSCs also couple to a mobile switching center (MSC) which interfaces to the Public Switched Telephone Network (PSTN) and other MSCs. Together, the BTSs, BSCs and the MSC form a wireless network, which provides wireless coverage to mobile stations (MSs) operating within a respective service area.
Wireless communication systems operate according to various protocol standards. One particular protocol standard in place worldwide is the CDMA protocol standard. CDMA is a direct sequence spread spectrum system in which multiple spread spectrum signals are transmitted and received simultaneously over a common frequency band. In the CDMA system, each mobile station (MS) may be assigned a distinct Walsh code, which identifies the signals, transmitted to and received from the MS.
In an example of operation thereunder, forward link signals from a BTS to a first MS are coded with a first Walsh code and then transmitted where the process of transmission includes pseudo noise (PN) scrambling (spreading). Likewise, forward link signals transmitted from the BTS to the second MS are coded with a second Walsh code and then transmitted, perhaps concurrently with transmissions from the BTS to the first MS. The first MS's receiver receives at its antenna all of the energy transmitted by the BTS. However, because Walsh code channels are orthogonal, after correlating the received signal with the first Walsh code, the despreader outputs all the energy intended for the first MS but none of or only a small fraction (i.e., due to orthogonality loss) of the energy intended for the second, third, etc., MS. Likewise, the second MS correlates the received forward link signal with the second Walsh code to receive its intended forward link energy. Each of the MSs then operates upon the despread signal energy to extract data intended for the respective MS. The number of users accommodated on the forward link is limited by intra-cell interference due to orthogonality loss, inter-cell interference and other interference such as that due to thermal noise.
In some specialized applications (e.g., fixed access) it may be considered to design a system such that orthogonal codes (e.g., Walsh codes) separate the signals of the reverse link users. However, for all CDMA systems currently deployed and reported in the literature, the reverse link is strictly interference limited, that is, one user's reverse link energy at the base station receiver acts as interference to other users signals. Thus, in the typical case, a plurality of MSs transmit to the BTS simultaneously on the reverse link with each reverse link transmission spread by a unique PN code or PN code shift. A receiver of the BTS receives the composite reverse link signal and despreads the reverse link transmissions with expected PN code to extract signals received from the first, second, third, etc., MS. The BTS then operates upon each despread signal to extract data sent by the MSs.
Wireless communication systems were originally designed and constructed to service voice communications. However, as packet data communications have increased in popularity, wireless communication systems have been called upon to service not only voice communications but packet data communications as well.
Examples of packet data communications supported by wireless communication systems include Internet sessions, electronic mail transfer, electronic file transfers, and short message services, among other services. Human users of the wireless communication system typically initiate these packet data communications. However, electronic devices may also access the wireless communication system, such devices including vending machines, credit card machines, ATMs and other computer controlled electronic devices. The wireless communication system provides a convenient and cost-effect method for sending packet data transmissions when a landline is not available.
Packet data communications place demands upon wireless communication systems that differ from those placed by voice communications. While voice communications require a substantially constant bandwidth, packet data transmissions are “bursty”, with high bandwidth required during some time periods and little or no bandwidth required during other time periods. In servicing either voice or packet data communications, limited channel resources must be assigned. The setup and initialization of a traffic channel in a CDMA system (for either packet data communications or voice services) requires between 20 and 30 frames. In a typical packet data session, once the traffic channel is setup, packet data transmissions between a MS and a BS will occur for a short period of time to complete the setup of the packet data session at all protocol levels. After the initial setup, the packet data transmissions are typically bursty, with periods of inactivity intermixed with bursts of data. Thus, the traffic channel may become idle for periods of time.
According to traffic channel management operations, when a traffic channel is idle for a period of time, e.g., one minute, the traffic channel is released. Thus, during a packet data session, when the traffic channel is inactive for the period of time, it is released. The release of the traffic channel does not, however, release the logical connection established for the MS via higher protocol layers (i.e., above the physical layer). For example, although the physical connection via the traffic channel may be terminated during periods of inactivity, the IP address of the mobile node, call control, and service information is maintained by the network. When the traffic channel is required again, it is again setup. This operation, in combination with the bursty nature of packet data communications requires the frequent allocation and release of traffic channels. Because the setup of a traffic channel consumes significant overhead (e.g., 20 to 30 frames of setup information on the traffic channel before transmission of user data), continued allocation and deallocation of traffic channels in servicing packet data communications is undesirable.
Many packet data communications transfer very little data after the initial setup of a logical link between a MS and a remote computer. For example, credit card verification services require the transmission of a relatively small number of frames of data. This amount of data does not justify the reallocation of a traffic channel since more frames would be consumed in setting up the traffic channel than would be used in transmitting the packet data. Thus, subsequent traffic channel allocation after the initial packet data communication setup of a logical link is not justified for this reason as well.
Responsive to these concerns, reverse link channels have been standardized for the transmission of reverse link packet data transmissions (and messages) from the MS to the BS. An example of an interference-based reverse link channel in a CDMA system is the reverse link access channel (R-ACH); a common channel that is defined within the IS-95 standard. The R-ACH employs the well-known Slotted Aloha protocol, where for IS-95 each slot is comprised of a preamble followed by a message capsule. The preamble is typically 3 to 4 frames and the message capsule is typically 4 to 10 frames. In transmitting packet data on the R-ACH, a MS simply initiates transmission, attempting to successfully complete the transmission to the base station without a collision (i.e., another mobile station trying to send a message during the same slot) from other MSs. Thus, some probability exists that the transmission will be received by the BS without contention from other MSs. The probability of successful transmission to the BS on the R-ACH decreases as the loading increases or with services that require more frequent access via the R-ACH such as telemetry, packet data, or credit card applications. Thus, as usage of the CDMA cellular system increases with new packet data applications the ability to effectively use the R-ACH decreases.
Another problem faced in using the R-ACH relates to the power control of transmissions on the R-ACH. The MS uses the received power of forward link transmissions to estimate the transmission power to use for reverse link transmissions (open loop power control). Based upon its estimate of the transmission power, the MS sends a transmission to the BS on the R-ACH. If the BS does not acknowledge receipt of the transmission, the MS increases its transmission power and retransmits. This process is repeated until the BS acknowledges the transmission or a maximum number of tries have attempted, in which case, the MS ceases its transmissions. Thus, fast closed loop power control does not exist for the R-ACH and the R-ACH channel requires a higher power for effective operation compared to channels which benefit from fast closed loop power control.
Thus, there exists a need in the art for a CDMA system, which has a viable mechanism for satisfying bursty packet data transmissions. Further such mechanism should satisfy the data transmission requirements of a MS without intervention of the BS. According to this mechanism, power control and collision avoidance should be managed on a reverse link channel for support of heavy cell loading and bursty packet communications on the reverse link.